1. Field of the Invention
The present invention is related to the control of microchannel processes, particularly microchannel processes which operate under generally high pressures and, optionally, generally high temperatures, and, more particularly, microchannel processes which comprise endothermic reactions such as steam methane reforming (SMR), and, optionally, exothermic reactions such as combustion. Control includes, particularly, methods of startup and shutdown of such processes.
2. Description of Related Art
Microchannel devices have demonstrated the capability of providing improved conversion of reactants to products as well as improved selectivity to desired products relative to undesired products and recent years have seen a significant increase in the application of microchannel processes to many unit operations. See, e.g., A. A. Rostami et al., Flow and Heat Transfer for Gas Flowing In Microchannels: A Review, 38 Heat and Mass Transfer 359-67 (2002) (applications in medicine, biotechnology, avionics, consumer electronics, telecommunications, metrology, and many others) and R. S. Wegeng et al., Compact Fuel Processors for Fuel Cell Powered Automobiles Based on Microchannel Technology, Fuel Cells Bulletin No. 28 (2002) (compact hydrogen generators for fuel cells). Microchannel processes utilize microchannel devices for carrying out unit operations that had previously been constrained to far larger equipment—often three to 1,000 times as large for comparable total throughput. Devices for microchannel processes, which microchannels contain features of at least one internal dimension of width or height of less than about 2 mm and preferably less than about 1 mm, have the potential to change unit operations in ways analogous to the changes that miniaturization has brought to computing technology. Microchannel processes can be used to advantage in small-scale operations, such as in vehicles or personal (portable) devices.
Importantly too, microchannel processes that can be economically mass-produced and connected together to accomplish large-scale operations are very desirable. For example, hydrogen gas is an important material in the operation of a petroleum refinery. The ability to economically generate hydrogen from a natural gas supply (i.e., methane) is important to such an operation and is typically effected, in part, via a reformer. In an SMR operation, for example, methane is catalytically reacted with water in the form of steam in the following reaction:CH4+H2O→3H2+CO.SMR being an endothermic reaction, a combustion reactor is often combined with the reformer to provide the necessary thermal energy. Notably, the reformer is operated at a temperature of about 650-1,000 deg. C. and a pressure of about 300 psig. Many microchannel devices utilized for unit operations such as SMR include a multi-planar design which then must operate in the high temperature and pressure regimes noted. Unlike a tubular reactor, a multi-planar device does not easily handle such pressures at the temperatures required.
Although not exclusively, these process units are typically constructed by laminating multiple planar sheets together where some sheets comprise openings which cooperate with other sheets to form microchannels. See, e.g., Schmitt, “Method of Fabricating Multi-Channel Devices and Multi-Channel Devices Therefrom”, U.S. Pat. No. 6,851,171 and Mathias et al., “Multi-Stream Microchannel Device”, U.S. Pat. Pub. No. 2004/0031592 A1. In addition to the structural integrity issues raised by planar elements and laminations and the temperature and pressure issues noted above, thin walls to reduce weight and improve heat transfer add further complexity. This is even more evident during startup and shutdown (both normal and emergency and including shutdown and subsequent “hot startup”) when temperature and pressure dynamics can be most difficult to control and which have the potential to damage the device or create hazardous conditions when flammable or potentially explosive mixtures are present. Thus, excess pressure differentials and uneven heating and “hot spots” in the device must be avoided or minimized.